Add support for NGSI-LD API

[bobeal]

(Quoting last feedback from @fgalan in #81)

Moreover, with regards to NGSI-LD, are you planning actual modification in the IOTA code itself? > As far as I understand, the NGSI-LD interface towards CB is part of the IOTA Node Lib work and > it's already done in a first experimental version available in this branch: https://github.com/telefonicaid/iotagent-node-lib/tree/feature/842_ngsi_ld. No modifications in IOTA code should be > needed.

Indeed, our current development is based on this branch of the IOTA Node Lib (the Sigfox agent already used the lib). We don't plan (nor expect) to do any specific modifications in the IOTA Node Lib (we'll just raise issues and / or fixes if we find something along the way).

The work-in-progress branch is here: https://github.com/easy-global-market/sigfox-iotagent/tree/feature/ngsi-ld (of course, it will be cleaned up before submitting a PR).

See also how other IOTAs are using this functionality (it's easy, just to use the branch as
dependency): telefonicaid/iotagent-ul#420

Ok, thanks for the pointer.

[fgalan]

I have had a look at master...easy-global-market:feature/ngsi-ld and it seems most of the changes are related with fixes in the code (e.g. adding the context parameter to log statements, unhardwiring port in tests, etc.). Or maybe I'm wrong?

If I'm right, it would be great to separate the work regarding bugfixing and the "pure" work on NGSI-LD in two separate PRs.

[bobeal]

Of course, don't worry.